Histomorphologic Profile of Myocardial Segments With Normal Thallium Activity in Chronic Ischemic Cardiomyopathy: In a subset of pts with chronic CAD and LV dysfunction, asynergic segments with normal thallium (Tl) activity may show lack of improvement in contractile function despite undergoing successful revascularization. Since quantitative analysis of Tl SPECT relies on normalized data, (in which the segment with greatest Tl activity is set to 100% and other segments are analyzed relative to this reference standard), we hypothesized that varying degree of interstitial fibrosis may explain the lack of postoperative improvement in such segments. To this end, we studied the histomorphologic profile in 13 chronic ischemic cardiomyopathy pts listed for cardiac transplantation who underwent stress-redistribution-reinjection thallium SPECT. The explanted hearts were sliced in short-axis sections (mean thickness 8 mm) and the volume fraction of collagen from midventricular slices were studied quantitatively using computerized videodensitometry after staining with picrosirius red. The corresponding tomograms for Tl-S, Tl-RD, Tl-RI were divided into 8 segments and the myocardial segment with maximum counts on the Tl stress image series was used as the normal reference segment for that patient. A total of 28 segments were identified to have normal thallium perfusion (<85% peak activity), with a mean of 96+6%. The volume fraction of collagen replacement in such segments ranged from 4.4% to 38% (mean 15.0+8.7%) which was significantly higher than the mean volume fraction of collagen obtained in 13 age-matched normal hearts without CAD (mean 4.1+1.7%, p<0.001). The distribution of collagen replacement among the CAD pts was due to a combination of interstitial fibrosis (n=27) and subendocardial (n=10) or diffuse (n=5) microscopic infarcts. In addition, despite exhibiting normal thallium activity, 12 of the 14 segments with <15% collagen volume were supplied by severely narrowed (<90% cross-sectional area luminal narrowing by pathology) coronary arteries. These histomorphologic data in myocardial segments determined to be normal by thallium provide a potential explanation for the apparent discrepancy between degree of viability by thallium and recovery of function after revascularization is a subset of pts with chronic CAD and LV dysfunction.